import java.awt.*;
import java.util.List;

public class ListNode {
      int val;
      ListNode next;
      ListNode(int x) { val = x; }
 }

class Solution {
    //解法1：迭代
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummyHead = new ListNode(-1);
        ListNode pre = dummyHead;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                pre.next = l1;
                pre = pre.next;
                l1 = l1.next;
            } else {
                pre.next = l2;
                pre = pre.next;
                l2 = l2.next;
            }
        }
        if (l1 != null) pre.next = l1;
        if (l2 != null) pre.next = l2;
        return dummyHead.next;
    }
    //解法2：递归
    public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val <= l2.val) {
            l1.next = mergeTwoLists(l1.next,l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1,l2.next);
            return l2;
        }
    }
}